In the description and the claims the term “aluminum p-doped” is used. It means p-doped with aluminum as the p-dopant.
The p-doping of n-type silicon by thermal diffusion of a p-dopant like boron into an n-type silicon substrate is well-known. Thermal diffusion is typically carried out using a diffusion source of the p-dopant, for example, gaseous BBr3. The p-dopant may be thermally diffused into a surface region of the n-type silicon substrate thus forming a thin p-doped layer with a low penetration depth of, for example, up to 200 nm. Said thermal diffusion process can be supported by masking certain portions of the n-type silicon substrate surface, i.e. those surface areas which shall not receive the p-dopant.
A solar cell is a particular example of a semiconductor.
A conventional solar cell structure consists of a p-type base with a front n-type surface (front n-type region, front n-type emitter), a negative electrode that is deposited on the front-side (illuminated side, illuminated surface) of the cell and a positive electrode on the back-side. Typically, the p-type base with the front n-type surface is p-type silicon with a front n-type silicon surface.
Alternatively, a reverse solar cell structure with an n-type base (n-type solar cell) is also known. Such cells have a front p-type surface (front p-type region, front p-type emitter) with a positive electrode on the front-side and a negative electrode to contact the back-side of the cell. Typically, the n-type base with the front p-type surface is n-type silicon with a front p-type silicon surface.
Other recent solar cell design concepts also include n-type bases, typically n-type silicon bases, wherein heterojunction p-type emitters are formed locally on the back surface of the solar cells. Here, positive as well as negative electrodes are located on the back-side of the solar cell.
n-Type solar cells can in theory produce absolute efficiency gains of up to 1% compared to p-type solar cells owing to the reduced recombination velocity of electrons in the n-doped semiconductor substrate.
The production of an n-type solar cell typically starts with the formation of an n-type substrate in the form of an n-type wafer, typically an n-type silicon wafer. To this end, an n-doped base is typically formed via thermal diffusion of a phosphorus containing precursor such as POCl3 into the wafer. The n-type wafer may have an area in the range of, for example, 100 to 250 cm2 and a thickness of, for example, 180 to 300 μm. On the n-type wafer one or more p-type emitters are formed via thermal diffusion of a boron containing precursor such as BBr3. The resulting p-type emitter(s) are either formed as a p-type emitter over the entire front-side surface of the n-type wafer, or as local p-type heterojunctions on the back surface. The p-n junction is formed where the concentration of the n-type dopant equals the concentration of the p-type dopant.
A dielectric layer, for example, of TiOx, SiOx, TiOx/SiOx, SiNx, Si3N4 or, in particular, a dielectric stack of SiNx/SiOx is then typically formed on the wafer to a thickness of, for example, 80 to 150 nm by a process, such as, for example, plasma CVD (chemical vapor deposition). Such a layer serves as an ARC (antireflection coating) layer and/or as a passivation layer.
A solar cell structure with an n-type base has one or more positive electrodes (either one on the front-side or one or more positive electrodes on the back-side) and a negative electrode on the back-side. The anode(s) is/are applied (typically by screen printing) from a conductive metal paste, typically a silver paste, and then dried and fired. A front anode is typically in the form of a grid or a so-called H pattern which includes (i) thin parallel finger lines (collector lines) and (ii) two busbars intersecting the finger lines at right angle. In addition, a conductive metal back cathode, typically a silver back cathode, is formed over portions of the back-side for interconnecting solar cells. To this end, a conductive metal paste, typically a silver paste, is applied (typically by screen printing) and successively dried on the back-side of the substrate. Normally, the back-side conductive metal paste is applied onto the n-type wafer's back-side as a grid, for example, an H pattern grid, or as two parallel busbars or as rectangles (tabs) ready for soldering interconnection strings (presoldered copper ribbons). The back-side conductive metal paste is fired becoming a back cathode. Firing is typically carried out in a belt furnace for a period of 1 to 5 minutes with the wafer reaching a peak temperature in the range of 700 to 900° C. The front anode and the back cathode can be fired sequentially or cofired.
MWT (metal-wrap-through) solar cells represent a special type of the aforedescribed solar cells. They have another cell design and they are also well-known to the skilled person (cf. for example, the website “http://www.sollandsolar.com/IManager/Content/4680/qfl7/mt1537/mi30994/mu1254913665/mv2341” and the leaflet “Preliminary Datasheet Sunweb” which can be downloaded from that website and F. Clement et al., “Industrially feasible multi-crystalline metal wrap through (MWT) silicon solar cells exceeding 16% efficiency”, Solar Energy Materials & Solar Cells 93 (2009), pages 1051-1055). MWT solar cells are back contact cells allowing for less front-side shadowing than standard solar cells.
Just like in case of the standard solar cells mentioned above, MWT solar cells can be produced as MWT solar cells having a p-type base (p-type MWT solar cells) or, in the alternative, as MWT solar cells having an n-type base (n-type MWT solar cells). Typically, the base material is silicon.
n-Type MWT solar cell wafers are provided with small holes forming vias between the front- and the back-side of the cell. The n-type MWT solar cells have a p-type emitter extending over the entire front-side and the inside of the holes. The p-type emitter is covered with a dielectric passivation layer which serves as an ARC layer, as is conventional for solar cells. Whereas the p-type emitter extends not only over the entire front-side but also over the inside of the holes, the dielectric passivation layer does not and leaves out the inside of the holes. The inside of the holes, i.e. the p-type diffusion layer not covered with the dielectric passivation layer, is provided with a metallization. The metallizations of the holes serve as emitter contacts and form anodic back contacts of the n-type MWT solar cell. In addition, the front-side of the n-type MWT solar cell is provided with a front-side metallization in the form of thin conductive metal collector lines which are arranged in a pattern typical for MWT solar cells, for-example, in a grid- or web-like pattern or as thin parallel finger lines. The term “pattern typical for MWT solar cells” means that the terminals of the collector lines overlap with the metallizations of the holes and are thus electrically connected therewith. The collector lines are applied from a conductive metal paste and they are fired through the front-side dielectric passivation layer thus making contact with the front p-type surface of the n-type MWT solar cell wafer.
The back-side of an n-type MWT solar cell is provided with cathodic conductive metal collector back contacts, which are in any case electrically insulated from the metallizations of the holes. The photoelectric current is collected from the anodic back contacts and the cathodic conductive metal collector back contacts of the n-type MWT solar cell.
Similar to the production of a standard solar cell of the reverse type mentioned above, the production of an n-type MWT solar cell starts with the formation of an n-type substrate in the form of an n-type wafer, typically an n-type silicon wafer. To this end, an n-doped base is typically formed via thermal diffusion of a phosphorus containing precursor such as POCl3 into the undoped wafer. Small holes forming vias between the front- and the back-side of the wafer are applied, typically by laser drilling. The holes so produced are evenly distributed over the wafer and their number lies in the range of, for example, 10 to 100 per wafer. Then a p-type diffusion layer (p-type emitter) is formed, typically via thermal diffusion of a boron containing precursor such as BBr3. The p-type diffusion layer is formed over the entire front surface of the n-type wafer including the inside of the holes. The p-n junction is formed where the concentration of the n-type dopant equals the concentration of the p-type dopant.
After formation of the p-type diffusion layer, excess surface glass is removed from the emitter surface by etching, in particular, in a strong acid like, for example, hydrofluoric acid.
Typically, a dielectric ARC layer, for example, of TiOx, SiOx, TiOx/SiOx, SiNx, Si3N4 or, in particular, a dielectric stack of SiNx/SiOx is then formed on the front-side p-type diffusion layer leaving out however the inside of the holes and, optionally, also a narrow rim around the front-side edges of the holes. Deposition of the dielectric may be performed to a thickness of, for example, 50 to 100 nm by a process such as plasma CVD (chemical vapor deposition) or sputtering.
Just like a standard solar cell structure with an n-type base, n-type MWT solar cells typically have a positive electrode on their front-side and a negative electrode on their back-side. The front anode takes the form of thin conductive collector lines arranged in a pattern typical for MWT solar cells. The thin conductive collector lines are typically applied by screen printing, drying and firing a conductive metal paste, typically a silver paste, on the ARC layer on the front-side of the cell, whereby the terminals of the collector lines overlap with the metallizations of the holes to enable electric connection therewith. Firing is typically carried out in a belt furnace for a period of 1 to 5 minutes with the wafer reaching a peak temperature in the range of 700 to 900° C.
As already mentioned, the holes of the n-type MWT solar cell wafers are provided with metallizations. To this end, the holes themselves are metallized by applying conductive metal paste to the holes, either in the form of a conductive metal layer (open holes) or in the form of conductive metal plugs (holes filled with conductive metal). The metallizations may cover only the inside of the holes or also a narrow rim around the edges of the holes, whereby the narrow rim may be present on the front-side edges of the holes, on the back-side edges of the holes or on both. The metallizations may be applied from one single conductive metal paste. It is also possible to apply the metallizations from two different conductive metal pastes, i.e. one conductive metal paste may be applied to the front-side of the holes and the other to their back-side. After application of the one or two conductive metal pastes it is/they are dried and fired to form p-type emitter contacts and, respectively, anodic back contacts of the n-type MWT solar cell. Firing is typically carried out in a belt furnace for a period of 1 to 5 minutes with the wafer reaching a peak temperature in the range of 700 to 900° C. The fired metallizations of the holes are in electric connection with the terminals of the thin front-side conductive collector lines.
In addition, a back-side conductive metal paste, typically a silver paste, is applied, typically screen printed, and successively dried on the back-side of the n-type wafer avoiding any contact with the metallizations of the holes. In other words, the back-side conductive metal paste is applied ensuring that it stays electrically insulated from the metallizations of the holes prior to as well as after firing. The back-side conductive metal paste is applied evenly distributed over the back-side of the n-type substrate, then dried and transformed by firing to become evenly distributed cathodic conductive metal back collector contacts. Firing is typically carried out in a belt furnace for a period of 1 to 5 minutes with the wafer reaching a peak temperature in the range of 700 to 900° C. The front anode, the metallizations of the holes and the back cathodes can be fired sequentially or cofired. The conductive metal back collector contacts account only for a small area of the back-side of the n-type substrate. In addition, the front-side conductive metal paste applied as thin collector lines fires through the ARC layer during firing, and is thereby able to electrically contact the front-side p-type emitter.